A registry cleaner is a software tool that helps to clean up and optimize the Windows registry. The registry is a database that contains all of the settings and configuration information for Windows and the various applications installed on your computer. Over time, the registry can become cluttered with outdated, unused, or incorrect information, which can slow down your computer and cause various errors and problems. A registry cleaner scans the registry for these issues and removes or repairs them, improving the performance and stability of your system. However, it is important to use a reputable registry cleaner and to be cautious, as some may cause more harm than good.